Rapid solidification (RS) technology in which alloys are rapidly splat quenched produce either amophous metals or metals with microcrystalline structure in which in the as-quenched state (AS), solid solution hardening increases the hardness of the various alloy compositions thus formed. While .beta.-titanium alloys are widely used in the aircraft industry for structural components, these alloys are not routinely used extensively in high temperature environments, 500.degree.-600.degree. C., for turbine blades and high temperature bearings. Thus, while .beta.-titanium-aluminium compositions have been used routinely for structural components, such alloys are not capable of being utilized in high temperature environments. On the other hand, .beta.-titanium alloys have been used in high temperature applications, although their use has been limited due to poor high temperature creep characteristics and other high temperature instability. .beta.-alloys are those alloys exhibiting hexagonal or near-hexagonal structures and are utilized in high temperature applications whereas the .beta.-alloys, exhibiting cubic structures, are utilized in the low temperature applications.
Rapid solidification processing (RSP) has proven that the microstructure and mechanical properties of RS alloys have improved a great deal through the processing as demonstrated in Fe, Al, based alloys and superalloys. Nevertheless, little study has been made in Ti alloy systems primarily because of their strong chemical reaction with crucible materials, which has been a stumbling block for the production of research materials. A recent study in this area includes synthesis of RS Ti alloys containing metalloids or pure Ti with rare earth metals (Y, Er) as additives. However, the role of rare earth metals in pure Ti is to form segregated particles or to give off rare earth oxide particles through the reaction with free oxygen.
More specifically, S. M. L. Sastry has reported extensively on the utilization of pure titanium with rare earth metals to provide rare earth oxides which segregate in spherical form within the titanium structure. See for example, "Dispersion Strengthening of Titanium Alloys," S. M. L. Sastry et al., TMS-AIME Fall Meeting, Oct. 25-28, 1982 in which pure titanium containing certain rare earth elements was rapidly solidified by electron beam melting and splat quenching. Note is also made of Sastry's work reported at the same conference in a Sastry paper entitled "Characteristics and Thermal Stability of Dispersed Phases in Rapidly Solidified Titanium-Rare Earth Alloys." Also noted is a paper to Sastry et al. in the 112th Annual Meeting, Mar. 6-10, 1983 of TMS-AIME entitled "Effect of Incoherent Dispersoids on Creep Deformation of Titanium." Moreover, early work of Sastry includes a paper entitled "Influence of Erbium and Yttrium Additions on the Microstructure and Mechanical Properties of Titanium Alloys," pp. 1185-1190, B. B. Rath et al., Titanium '80 Science and Technology, Proc. IV Conf. on Titanium, May 1980, in which no rapid solidification technology is involved. Sastry's contributions are thus limited to the use of pure titanium to which rare earth elements are added, there being no description of age-hardening due to the lack of chemical reaction between titanium and the rare earths.
More recently it has been reported by Whang et al. in the aforementioned 1982 TMS-AIME Fall Meeting and by Y. Z. Lu and S. H. Whang at the Fall TMS-AIME Meeting in October 1982 that silicon while used in titanium to increase creep resistance, coarsens very quickly such that particles one micron in diameter are produced which result in very little if any strengthening of the material. Moreover, although the addition of silicon to the titanium produces a modest strength improvement, it does not provide an alloy system suitable for use in high temperature applications. High temperature applications include the utilization of these alloy at temperatures exceeding 500.degree. C.
As described by Peng et al. at the TMS-AIME 112th Annual Meeting of Mar. 6-10, 1983 in a paper entitled "Microstructures and Mechanical Properties of Rapidly Solidified Ti-B and Ti-6Al-4V-B Alloys," the use of vanadium with boron produces alloys of titanium boride. However, these alloys are not usable in high temperature applications due to the needle-like martensitic structures produced.
Finally, as reported in Materials Science and Engineering, 23 (1976) 135-140, R. Wang describes pure titanium as an additive to rare earth as opposed to rare earth as being an additive to titanium.
It should be noted that titanium alloys have been traditionally strengthened by a solid solution strengthening method in which the strength of the material increases up to a certain temperature at which point it falls off rather rapidly. Thus, solid solution strengthening is not effective at the high temperatures associated with certain aircraft requirements. Titanium strengthening has also been occasionally accomplished by RSP technology along, but this strengthening has not been significant. Moreover, while "so-called" superalloys exist which provide the requisite strength up to 90% of the melting temperature alloy, there are no known superalloy-like compositions for titanium and no known reported precipitation alloy hardening techniques for titanium structures.
In order to provide for high temperature titanium alloys, there is therefore a need for precipitation hardening in which the precipitates or dispersoids, if spherical, if small enough, and if equally distributed, provide barriers which prevent dislocation movement such that the strength of the material is increased. This being the case, the final alloy is provided with much increased strength at high temperatures since the particle effectively blocks dislocation movement. In order to provide a stable precipitation to prevent movement of dislocations, the precipitate must be fine, on the order of 50-100 Angstroms in diameter, uniformly dispersed throughout the material, spherical in structure, and of high density.